When the switches of a power factor corrected switched mode power supply are switched at a constant frequency there is a significant EMI noise signature at certain frequencies. It is known to modulate the switching frequency applied to the switches to spread the noise spectrum over a wider range. When compared to an un-modulated switching frequency, this reduces the average noise energy.
FIG. 1 shows a prior art approach to modulating the switching frequency based on the phase of the rectified power supply (as per the application note AN833 for the L4981B integrated circuit produced by STMicroelectronics). Curve 1 shows the mains waveform and line 2 the switching frequency modulation. As shown in FIG. 1 the switching frequency is reduced to 80% at the point of maximum power transfer and increased to 100% at minimum power transfer. This approach can reduce EMI noise by about 3 dB. The problem with this approach is that the rate of change of the switching frequency is minimized at the point of maximum power transfer and so much of the EMI noise power is confined to a narrow range.
It would be desirable to provide a power factor corrected switched mode power supply having reduced EMI noise or to at least provide the public with a useful choice.